1. Field of the Invention
The present invention relates capacitors, and more particularly a parallel plate capacitive structure for connecting to microwave coaxial transmission cables.
2. Description of the Related Art
FIG. 1A illustrates a microwave coaxial transmission cable 100 that has an inner conductor 102 and an outer conductor 106. FIG. 1B shows a cutaway side view of microwave coaxial transmission cable 100. Inner conductor 102 and outer conductor 106, which are parallel to one another, are separated from one another by an insulator 104 that surrounds inner conductor 102. An additional insulator 108 (also known as a shield) surrounds an outer surface of outer conductor 106. It is noted that the thickness of each of elements 102, 104, 106 and 108 of cable 100 are not necessarily drawn to scale in FIGS. 1A and 1B.
It is sometimes desirable to block low frequencies from being propagated through a microwave transmission line made up of one or more microwave coaxial cables 100. Capacitors are typically used to block low frequencies. However, the use of a capacitor in a coaxial environment presents challenges due to the size and structure of coaxial cables 100. Coaxial cables are typically connected to a substrate containing one or more capacitors, and a transition is made back from the substrate to another coaxial cable.
In accordance with an embodiment of the present invention a capacitive structure is provided including first and second parallel plate capacitors. The first parallel plate capacitor includes an upper conductive plate and a lower conductive plate that are substantially parallel to one another and separated from one another by a first dielectric material. Similarly, the second parallel plate capacitor includes an upper conductive plate and lower conductive plate that are substantially parallel to one another and separated by a second dielectric material. The lower conductive plate of the first capacitor is engaged against, and thereby connected to, the upper conductive plate of the second capacitor. A conductive clip connects the upper conductive plate of the first capacitor to the lower conductive plate of the second capacitor.
The conductive clip includes a body having an outer surface and an inner surface, and the clip body is substantially rectangular. The inner surface includes a channel to prevent the lower conductive plate of the first capacitor and the upper conductive plate of the second capacitor from shorting with the upper conductive plate of the first capacitor or the lower conductive plate of the second capacitor. The channel extends a width of the body of the conductive clip. A height of the channel is preferably greater than a combined height of the lower conductive plate of the first capacitor and the upper conductive plate of the second capacitor to prevent contact with these plates, which would result in shorting.
Extending from opposite ends of the inner surface are a first lip and a second lip, which form a cavity for accepting the first and second capacitors. When the capacitors are within the cavity, the first lip engages against an outer surface of the upper conductive plate of the first capacitor, and the second lip engages against an outer surface of the lower conductive plate of the second capacitor.
Electrical contact is made between the clip and a first coaxial cable to enable a signal to be transmitted through the capacitors. The outer surface of the conductive clip body includes a first bore to receive a first inner conductor of a first coaxial cable to make the contact. The bore extends into, but preferably not through, the body.
To make contact with a second coaxial cable, a conductive cup is connected to the lower conductive plate of the first capacitor and the upper conductive plate of the second capacitor. The conductive cup includes a second bore to receive a second inner conductor of the second coaxial cable. To maximize electrical contact, a diameter of the conductive cup is preferably greater than a combined height of the lower conductive plate of the first capacitor and the upper conductive plate of the second capacitor. Additionally, to prevent undesired shorting of the plates of one capacitor together, the diameter of the conductive cup should be less than a combined height of the first dielectric material, the lower conductive plate of the first capacitor, the upper conductive plate of the second capacitor and the second dielectric material.
In an embodiment of the present invention, to assure contact is maintained between the parallel plate capacitors and the first and second coaxial cables, one or both of the first inner conductor and the second inner conductor includes a central bore, within which an axial pressure contact member is inserted. A tapered end of the axial pressure contact member rests within the corresponding bore of the conductive clip and conductive cup (i.e., the first bore of the clip, or the second bore of the cup), and provides axial pressure to keep the capacitive structure firmly between the first and second inner conductors.